1. Field of the Invention
This invention pertains generally to an implantable medical lead for stimulating muscle contraction, and specifically to a paraneural electrostimulating bipolar shielded lead for use in surgical procedures such as the Muscle Flap Heart Function Augmentation Procedure and/or other circulatory augmentation procedures powered by skeletal muscles.
2. Description of the Prior Art
Congestive heart failure represents a pathophysiologic state in which cardiac output is inadequate to meet the demands of a body. Reduced cardiac output results from many known conditions and diseases, including ventricular tumors and/or aneurisms, stenosis or other diseases of the valves, cardiac muscle damage due to myocardial infarction, toxic or inflammatory diseases, and the like. Cardiac transplantation and the mechanical heart are among the most recently developed invasive methods of improving cardiac output. While clinical results with cardiac transplantation have been impressive, progressive cardiac deterioration frequently results from localized cardiac muscle damage which, at least in theory, would not terminate requiring cardiac transplantation if alternative methods of cardiac assistance and/or reconstruction were available. Further, the limited availability of matching or compatible donor organs significantly restrains the number of patients who can benefit from this procedure. Mechanical circulatory support devices have been used to assist failing ventricles, to serve as a bridge for cardiac transplantation, and to provide limited-term circulatory support. However, the complex mechanisms of power supply, the prohibitive costs, and the complications of thromboembolism and infection have limited the success of the mechanical heart.
Just as prostheses were developed to repair damaged or diseased cardiac valves, similarly, a significant need persists for a method of reconstructing and/or supplementing cardiac muscle in the event of localized damage such as ventricular tumor or aneurism or some forms of global abnormality.
In order to meet this need, a surgical procedure known as Muscle Flap Heart Function Augmentation which is capable of increasing cardiac output was pioneered by George Magovern, M.D. This procedure is also known as cardiomyoplasty and is disclosed and claimed in U.S. Pat. No. 4,791,911 issued Dec. 20, 1988 to George Magovern, M.D.
This surgical procedure utilizes the principle that both cardiac and skeletal muscle are composed mainly of contractile proteins that can transform chemical energy into mechanical work. The myocardium is by nature a highly fatigue resistant and oxidative metabolizing muscle that is capable of continuously pumping blood for a lifetime. It has been found that with proper training through electric conditioning, skeletal muscle fibers can be transformed so that they function like those of the myocardium.
In the cardiomyoplasty surgical procedure, a patient's either (or both) latissimus dorsi muscle(s) is (are) dissected from its (their) natural location in the back. The blood supply and nerve tissues are not divided and remain intact to nourish and excite the muscle. The dissected muscle (also known as and hereinafter referred to as the "muscle flap") is translocated by passing it into the thorax through a partial rib resection below the axilla. After appropriate midsternotomy, cardiac surgery (and cardiopulmonary bypass when needed) etc., the muscle flap is used as a graft in the reconstruction of cardiac walls or wrapped around the ventricle(s) as a reinforcing structure.
For several days post-operatively, the muscle flap is permitted to heal in its new anatomical position before stimulating contractions (which might otherwise tear the sutures) are initiated. Subsequently, a protocol of suitable electric stimulating of the muscle flap is initiated, which enables the muscle flap to contribute to the overall cardiac function and cardiac output once the muscular fibers have been "trained" or "conditioned".
It has been found that the muscle flap can be made to beat in synchrony with the heart in response to signals generated by a type of cardiac pacemakers and/or a type of implantable pulse generator (IPG) as hereinafter described and generally known as a cardio-muscle stimulator, although it may be known by other names or terms as well. Further, as used herein, the term cardiomuscle stimulator is defined to include one or more devices, elements or any combination or configuration that is implanted for the purpose of stimulating live graft tissue to augment cardiac or circulatory function, including, but not limited to, co-pulsating and counter-pulsating devices. One example of such a device is Model SP1005, marketed under the trademark Cardio-Myostimulator.TM. available from Medtronic.RTM., Inc., of Minneapolis, Minnesota.
The cardio-muscle stimulator is both a "receiving" and a "sending" unit As the heart muscle begins its contraction, a sensor which is associated with the heart, senses the occurrence of electrical impulses generated by the heart muscle (specifically the depolarization of the atria or ventricles), which immediately precedes and initiates systolic contraction. The sensor in turn transmits a signal which is "received" by the cardio-muscle stimulator. The cardio-muscle stimulator in turn transfers that signal to its "sending" unit portion. The muscle channel, or sending unit portion of the cardio-muscle stimulator, generates stimulating pulses individually, or in a burst pattern, and--through a combination of stimulating leads which include electrodes implanted in the muscle flap--causes the muscle flap to contract in synchrony with the heart to which it is attached, assisting the heart and causing it to increase its output of blood to the body. The cardio-muscle stimulator is usually but not necessarily implanted in the upper left portion of the abdominal wall.
To understand the present invention, it is important to have an understanding of the association of the stimulating leads and electrodes with the muscle flap, and the process by which a signal generated by the cardio-muscle stimulator will cause a muscle flap to contract.
The latissimus dorsi of an adult is typically approximately 30-40 cm. long by 15-25 cm. wide and varies in thickness throughout its length. The general structure of such a flap is depicted in FIG. 1, and it will be helpful to review FIG. 1 before proceeding further. As depicted in FIG. 1 (discussed in greater detail, infra), the neurovascular bundle, in this case, the thoracodorsal bundle, comprising an artery, a vein, and a nerve, enters the head of the muscle flap, and similarly to a tree trunk and several branches, the neurovascular bundle ("trunk") subsequently branches into ever smaller dimensions as it permeates the entirety of the muscle flap.
The output signal of the cardio-muscle stimulator is transferred via stimulating leads to the muscle flap by placing the electrodes of the stimulating leads in contact with the muscle flap in the vicinity of the main nerve and/or major branches.
More specifically, the current art utilizes separate positive and negative electrodes placed within approximately 6 to 12 cm. of each other in the vicinity of the main nerve and/or major branches. While a single electrode could theoretically be used, with current returning to the case of a cardio-muscle stimulator which design allows it to be an electrical current ground, the use of two electrodes (comprising a positive and a negative) is preferred because this provides an efficient path for the current to travel resulting in a more efficient stimulation of the structure of the muscle flap. In the standard two-electrode system, current travels from the cathodic to the anodic electrode.
It should be noted that the stimulation of the muscle flap proceeds via the process known as "neuromuscular distribution"--wherein some muscle fibers (along with the main nerve branches) are electrically stimulated, which in turn carry the stimulus chemically or electro-chemically throughout the network of nerve branches which permeate the muscle flap, causing the fibers of the entire muscle flap to contract in response to the stimulus. This process should be distinguished from the stimulation of a single nerve branch in order to produce contractions of limited portions of muscle tissue. Further, it should be noted that the electrode portions of the stimulating leads are not placed in direct contact with the nerve, but are associated with the muscle tissue itself near the main nerve branches.
The use of medical leads is known, and certain terms common in the art connected with those leads/electrodes are generally defined and understood herein as follows:
"Unipolar" refers to leads having a single electrode, which could be either a negative (-) (cathode) or a positive (+) (anode) but not both. Typically, two unipolar electrodes (a positive electrode and a negative electrode) are required to achieve stimulation of the muscle flap;
"Bipolar" refers to leads which have both a positive (+) (anode) and a negative (-) (cathode) portion integrally formed in a single stimulating lead;
"Shielded" refers to a lead wherein the cathodic and/or anodic electrodes are in part covered with electrical insulation--permitting current to be released to or from the anode and/or cathode only in a specific limited direction, as in some limited portions of a plate, a rod or a sphere, as opposed to an unshielded electrode as defined below;
"Unshielded" when used in relation to an electrode, indicates that whether it is the anode and/or cathode, it is not covered by electrical insulation over any portion of its surface and may therefore release current in any and/or all directions along the exposed plate, rod or sphere. It is to be understood that "over any portion of its surface" means that portion of the electrode in contact with nerve or muscle tissue, it is being further understood that the portion of the lead not in contact with live tissue may be electrically insulated ("shielded") to prevent undesirable stimulation of structures located between the cardio-muscle stimulator and the muscle flap;
"Electrode" means the potentially active, non-electrically insulated portion of a lead and is further used as a general term which can mean either an anode and/or a cathode;
"Stimulating lead" refers to a lead which conducts electric stimuli from a power source to the receiving tissue, and incorporates all components of the lead from the point of attachment to a stimulating power source to its distal end, including the electrode(s). The stimulating lead may be used as a pacing lead;
"Pacing lead" refers to a lead which is similar in design to a stimulating lead except that the purpose of the pacing lead is usually directed to cardiac pacing.
It is to be noted that while the terms "pacing lead" and "stimulating lead" are nearly identical, and in fact are often used interchangeably in the art historically the term "pacing" has evolved in connection with the rhythmic pacing of cardiac tissue. While the pacing of cardiac tissue and the stimulation of non-cardiac muscle share some areas of technical overlap, there are also very significant technical differences associated with each of these procedures, therefore, the term "pacing" will more correctly identify the stimulation of cardiac tissue, whereas the term "stimulating" will more correctly identify the stimulation of non-cardiac tissue, specifically of skeletal muscle, which is being used to augment cardiac or circulatory function.
Additionally, there are several types of leads which lend themselves to various means of attachment to body tissue, which include the following:
"Screw-in" or "corkscrew lead" refers to electrodes having a screw or spiral-type configuration which is screwed into the tissue;
"Stab-in lead" refers to the method of electrode attachment wherein the electrode is positioned over the cardiac or skeletal muscle tissue and is implanted therewith by utilization of point and barb system similar to that of a fishing hook;
As used in conjunction with the concept of electrode attachment to body tissue, "suture lead" refers to a means of attaching the lead to body tissue and the lead is equipped with points of attachment which permit the electrode to be fixed with stitches to the tissue;
"Tined lead" refers to a lead with one or more tined portions, similar to a grappling hook, which is capable of affixing itself to the tissue in the same manner as a grappling hook attaches to various items as distinguished from the point and barb method of attachment of the stab-in type method of attachment;
"Wrap-around lead" refers to a lead which is physically wrapped around the tissue to be stimulated (which may be unipolar or bipolar in configuration);
"Intramuscular lead" refers to a lead which can be routed through the muscle flap tissue.